I thought this might be useful here, as I just posted it in response to an old thread over on DJforums.
I have not listened to the new Denons. (see updates on this way below) The old Denons sound very good. SPDIFs are probably sample rate converted linearly and poorly rather than the same exact rate/depth as in the unit, but the analog outs good on them.
The Hanpins' (ADJ, Stanton, Omnitronic, Citronix, DJ Tech, Reloop, Akiyama, etc) sound quality is a little lower fidelity from the digital outs compared to the Pioneers' digital outs, which are bit-perfect at zero pitch regardless of whether master tempo/key is on or off. Neither Hanpins nor Pioneers matter whether key lock is on/off as long as pitch is at zero. Hanpins are not bit-perfect, even at zero pitch. Obviously, if you're going to have pitch away from zero, having key lock off will sound better, particularly in the negative. Hanpins do have better pitch resolution on Wide than new Pioneers (0.1 vs 0.5), matching the old CDJ-1000mk3. The Hanpins do not have internal sound cards, but I am doing this comparison using USB thumb drives.
Gemini MDJ-1000, which has the old Denon-style Link and pitch resolution halfway between Hanpin and Pioneer, sound slightly more dry, less musical, and less liquid compared to old Denon, Pioneers, and Hanpins over SPDIF. This is either a clock issue or solely inside the firmware; I don't know which. Processing end I'm talking about, not analog outs. The Gemini MDJ-1000 tests fall between the Pioneer's (at zero pitch) flawless bit-perfect and the Hanpins' when the Gemini has key lock off and pitch at zero, but I still favor the Hanpins with key lock off to the MDJ... more liquid and pure in the mids. This is listening with four different models of digital DJ mixers with digital inputs (Denon, Pioneer, Numark, and Rane), listening direct into an install DSP, and testing with RMAA. I think the Gemini may have slightly better-sounding key lock in-use away from zero than the Hanpins, but with key lock on and zero pitch, Rightmark audio analyzer shows weird, mucked up results on the Gemini. So key lock on is always heavily processing the sound of the Gemini currently even more than it already does. The key lock off processing that is always present on the Gemini might be less harmonic & intermodulation distortion than the Hanpins, but I suspect it is odd-order, amusical distortion. Thus it could have lower level distortion and still sound worse. Or it could just be clock issues and jitter I can't measure for.
As for the analog outs, I'd put the Pioneer and old Denons at the top, followed by the Gemini, and then the Hanpins. I mean the ability to transparently reproduce the signal as close to what the SPDIF connection on the same unit can do bypassing DA/AD conversions, not how good their own internal processing is affecting the sound, which I have already gone over above. So I am not so much comparing their analog outs between players above, just trying to evaluate how good the analog section is on the player. Now, I did think that in absolute terms the old Denons sounded warmer than the Pioneers, but I didn't have a problem with any of the Pioneers and actually like (!!!) the sparkle and tone of the CDJ-1000mk3 and CDJ-900. The original CDJ-1000 and mk2 versions were definitely inferior to the Denons, and the mk3 and CDJ-900 do sound different than the old Denons too, but I don't think it's a bad thing, just a different tone from the analog outs. YMMV. It's really getting into the weeds and differing tastes. It's similarly a toss-up between the lower-tier Gemini and Hanpin units using the analog outs, as you've got worse processing but better analog output section on the former and the reverse situation on the latter. I don't think you can go wrong on the sound of the Pioneer and old Denons, and on the other two brands you are losing a little bit on fidelity. Sad, but true.
None of the above will allow you to lock key away from zero, as you can do in software and as could be done on the old Numark Axis series (Elliot Marx is a genius). Denon/InMusic has stated their new players might have this feature added later. It's useful not just for key-matching/compatibility purposes, but also for preserving sound quality by locking to the nearest adjacent key to where the pitch is at away from zero, minimizing key lock effects while still utilizing it.
FYI, all these players when not glitching up or malfunctioning (cough, Gemini) sound better than the average turntable setup used by the average vinyl DJ ever that is far from an audiophile experience.
I should also point out that the Gemini firmware is still a work in progress and beta quality, in my opinion, though they are making some progress. I hope that the sound can also be improved. Firmware 6.4 & 6.5 appear to have deactivated the sound cards on them, possibly as they attempt to improve the stability in standalone mode.
Also not sound related but does affect the DJing... Old Denon and Gemini have better jog bends than the Hanpins and Pioneers, though I will admit to preferring slightly now the Hanpin time-based jog bend over the Pioneer jog bend with the deadzone. My favorite is rate-based jog bend, with adjustable sensitivity and no deadzone, but time-based is something you can get used to and does have its advantages. Most of the Hanpins have overly sensitive jog bends and this cannot be adjusted.
The new Denons use similar AKM DACs and quality of analog out circuitry as the CDJ2000NXS2, but its capacitive touch screen makes them more susceptible to interference from unshielded speakers and smartphones. New Denons (original firmware only) have the best pitch resolution ever seen on a standalone USB player (near theoretical limits of 14bit pitch faders, similar to Hanpins in VDJ). The key lock/master tempo is also supposedly the best of any player yet released, but this is word of mouth and promotional material.
Update 1: Though they might impart a teeny tiny bit of interpolation texturing that wasn't there in the original signal, the old Denon digital DJ mixers have much better processing than the Pioneers. Not even close. 44.1khz mode is pretty darn transparent to the original signal, but 96khz resampling mode on a Denon seems to make what you put in sound more like music. So much goosebumps on the skin and butterflies in the stomach it's hard to mix and not just listen. The closest thing I can think of is listening to Burmester front end stuff at Alexis Park in Vegas years ago.
Rane MP2015 is probably the most ruthlessly revealing and detailed DJ mixer I've ever heard, but might slightly exaggerate some of the detail and blacken backgrounds beyond the reference signal. Closest description I can think of is like a grit-free, improved imaging, and even more detailed, digital, rotary version of a Biamp SCM in its digital in/out sound. Very special achievement in resolution, tightness, and precision.
Numark PPD9000 is wonderfully liquid and deep, but lacks the detail of the Rane or the sheer cohesive musicality of the Denon (possibly from that pleasing resampling interpolation). Side note: PPD9000's optional tube mode also seems interestingly useful for compensating for bad room acoustics at loud volumes and the Gemini dry sound, which I did not expect. Possibly related to the loudness curve equalization on it combined with added pleasing even-order harmonics that masks (to our ears) other forms of distortion.
The DJMs seem tailored more for dubstep and commercial EDM than for music with instruments and voices, as they can come off as bloomy in the lows, glassy in the mids, and crunchy in the highs, giving them a distinct sound compared to the others. I agree with the MOS sound guy who thought the lower mids are warm, but I don't think its a good warmth. Seems like some phasing issues or grunge going on in that band that prevents that and the lower band integrating with the rest of the mids. Ditto with the highs not sounding quite integrated into the mids. Works for some stuff; doesn't for others. I'd still rather listen to the others than a DJM, but it's not outright bad. Might beat most analog mixers ever made, but not these other stellar digital ones.
The analog output stages on all of them are not particularly inferior to the others, though, so any degradation on their overall analog output sound from one to another is mostly the result of their internal processing. The Rane analog input section appears to be unique in that it uses finite impulse response filters on its AD converters, like some high-end outboard digital signal processors. That said, again I think the biggest difference in sound on these digital mixers, ignoring the phono inputs, is on the internal processing, not from the line-level inputs/outputs. That surprised me.
Update 2: Oh, and I used the DB2 one night on the system I manage at a venue, and I thought it sounded nice using just the analog inputs and then out digitally into the processor. I have not had a chance to compare it or the DB4 directly with the others, though, let alone with the digital ins. It seemed neutral to warm, punchy, inoffensive, open, and effortless. There were many compliments that night, but I had the delays all set up kind of funky.
Update 3: The new Denons:
So old Pioneer DACs were ok, but new Pioneer DACs are great. Pioneer general audio processing independent of its keylock and its SPDIFs have always been reference that I can tell.
The old Denons always had great DACs, but simple not-the-best SPDIF. Keylock off processing seems fine.
New Denons have really awful general audio processing independent of the Elastique keylock and heavily resample all the audio internally rather than just putting it at the rate of the track being played like Pioneer does, and it needs to be converted again for the 24/96 SPDIF it seems. The DACs being fed the probably-wildly-oversampled internal processing that's likely higher than 24/96 on the new Denons are great, though. Whether the DAC sound is good enough in a user's opinion to offset the earlier processing degradation is a matter of opinion. Into high-end analog mixers, the Prime players sound pretty good, though, and I suspect it'd benefit from tubes to mask some of the nonlinear harmonic & IMD distortion on them with additional tubey distortion. The combination of resampled SPDIF, earlier processing degradation, and digital mixers being very revealing probably isn't the best combination. The new X1800 mixer, by the way, has worse digital to digital audio processing quality than the old X1700, too. Hopefully this all gets fixed in firmware updates.
I'd probably put the Gemini MDJ with keylock off ahead of the Hanpins at zero pitch in terms of sound quality, and the Gemini seems to do keylock a little better than the Hanpins. The Gemini has some resampling going on internally and its SPDIF is simple 16/44.1 linear resampled again like old Denon's. Gemini's analog outs, like the Hanpin DACs, aren't going to knock your socks off. So on the Gemini it's kind of a toss up as to whether the SPDIF or the DACs are a better choice. On the Hanpins, you should use the SPDIFs when you can as they're not making the internal processing sound on them any worse.
Oh, an update on the mixer side, too, I finally did get a DB4. Top tier I'd say is the X1700 for musicality and MP2015 for tightness & detail. DB4 is close behind, and is at least as high fidelity as the DJM900NXS2. The DB4 sound is highly customizable, though, in a way that the others are not. It can be a lot to handle, though.